In one type of communication system, character information is transmitted in the form of digital coded signals and received by sampling the transmitted digital coded signals with sampling clock pulses, the sampled signals being used for reproducing character information on a television screen.
As an example, in a Japanese teletext system, the specification for one packet of transmitted data calls for a 16 bit clock run-in pattern "1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0", an 8 bit framing code "1 1 1 0 0 1 0 1" and 272 bits of character data, as shown in FIG. 1, and this packet is transmitted in one horizontal period of the vertical retrace interval in a conventional television signal. The framing code is used as a start signal for character data, i.e., processing for character data is started immediately after detection of the framing code preceding the character data.
The framing code is defined in accordance with the specification for the particular teletext system, such as "1 1 1 0 0 1 0 0" for an English teletext system and "1 1 1 0 0 1 1 1" for a North American Broadcasting Teletext system. In an area where different teletext specifications are used, it is necessary for proper processing of teletext data to detect the originating teletext specification for the received data, since character data formats are different in accordance with the teletext specification. For this purpose, it seems to be effective for detection of the teletext specification to identify the framing code.
In Japanese published patent application No. 2230/1982, a framing code detector is disclosed which comprises a shift register for converting a received serial framing code into a parallel bit signal, a memory for producing an output signal only when a predetermined address location corresponding to a specified framing code is accessed, and means for supplying said parallel bit signal as an address signal to said memory. However, if the above detector is used for a multi-teletext receiver, the number of detectors provided in the multi-teletext receiver equals the number of framing codes to be detected. This leads to an increase in circuit structure which causes an increase in system cost and a greater possibility of misoperation of the system.